JP 2014-225942 A discloses an energy storage system for starting an engine of a vehicle. This energy storage system uses, in place of a lead-acid battery, an energy storage device (secondary battery), such as a nickel hydrogen battery or a lithium ion battery, which is lower in charge voltage than a lead-acid battery.
The energy storage system of this type uses a control method different from that of a lead-acid battery. Since this energy storage system is mounted to a vehicle, the outer shape thereof or the shape of a communication connector thereof are similar to those of the lead-acid battery. If the energy storage device (secondary battery) having lower charge voltage than a lead-acid battery is mounted, by mistake, to a lead-acid battery vehicle, this energy storage device is charged similarly to the lead-acid battery. Then, the energy storage device is overcharged to cause some troubles.